There are two types of grammarians, or so I'm told. Actually, there's only one, but I'll get to that.
Types of grammars can be labeled either descriptive or prescriptive. A descriptive grammarian is one who writes rules for the usage of a language based on how he hears it being used by the people around him. A prescriptive grammarian writes rules based on how the language should be used, whether the people around him use it that way or not. A descriptive grammarian tells us how the world is; a prescriptive grammarian tells us how it should be.
Now, you already know I'm a grammar snob, so which type of grammar do you think I prefer? Prescriptive of course. For one thing, I couldn't care less how the people around me speak English. What if they're in the third grade, or illiterate? My high school German teacher, the beloved late Marty Deweese, once told her class of her earliest introduction to German, when she babysat a toddler for some friends of hers who were living in Germany at the time. She took the toddler on a train and he pointed at various farm animals and told her the names of each of them, in German. She was very excited to know these new German words, and repeated them to her friends when they returned. But she was crestfallen to discover that the new words she learned, translated into English, became "moo-cow" and "cluck-cluck bird" and "run-dog" and so forth. Imagine a descriptive grammar based on this three-year-old's vocabulary.
The difference between the three-year-old and some Americans is one of degree, not kind. So much has been written about the death of the semi-colon that I don't know why we still put one on computer keyboards, although it's probably useful to software engineers. I've already spoken about the transformation (read "degradation") of the word "loan" from a noun to a verb. Maybe that's a subtle point, but what about more widespread and egregious language abuses? There are large numbers of native English speakers in this country who would not see or hear a problem with the sentence "I seen him yesterday". Or "we was down at the store". A descriptive grammarian, writing a new textbook on sentence structure based on these sentences, would have no need for a chapter on noun-verb agreement. Now, she might argue that each speaker's meaning is perfectly clear. And, in fact, the speakers would be communicating. For example, someone who says "I seen him yesterday" has told me that he saw him yesterday. He's also told me that he's a hick. Usage does more than forward data to the listener; it conveys the education and sophistication of the speaker.
Of course, maybe part of the problem is we live in a time and society where education is no longer valued. When movable print was invented, the educated class (i.e., the clergy) worried that anyone could publish a book. The English language managed to survive that crisis (or, I should say, German did, but you get the point.) The invention of the typewriter was accompanied by a similar alarm, but language weathered the storm. Similarly, the electric typewriter, the word processor, the personal computer, and the laser printer made it easier for people with less and less education to produce documents with higher and higher visual appeal, even though they might be riddled with errors. But now we have blogs, and webpages, and people who either don't care about their atrocious spelling, grammar, usage, punctuation, or style, or don't realize how badly they write. "Descriptive" grammarians are of no use when it comes to correcting this backslide. Instead, in two decades the ideas of proper case, helping verbs, subject-verb agreement, and others will only be of interest to historians.
But not if the real grammarians have anything to say about it.